This application is related to the concurrently filed Cheung application Ser. No. 752,011 entitled "Expanding Strap Loop Forming and Friction Fusion Machine."
In the past, Signode Corporation, the assignee of the entire interest of the present application, has developed several processes and machines for forming a strap loop about a package, tensioning the loop, and joining the overlapping portions of the tensioned loop.
Some strapping machines are portable and of the semi-automatic type. For example, the operator must first loop the strap about the package and then the machine is applied to tension, seal, and sever the loop. Other machines, such as the machine disclosed in the Kobiella U.S. Pat. No. 3,442,203, are of the completely automatic type, i.e., one which automatically feeds a thermoplastic strap around a package from a strap supply source, grips the leading end of the strap, withdraws the standing length of the strap to tension the strap loop, friction-fuses the overlapping portions of the loop, and severs the loop from the standing length of the strap. This type of automatic strapping machine has a relatively large, ring-like, rigid chute into which the package is inserted and in which the strap is fed to form a closed loop around the package.
Strapping machines are available today for joining the overlapping portions of a tensioned loop in a number of different ways. The overlapped loop portions may be secured with an independent seal which is placed around the overlapped strap portions and clamped or crimped thereto. A second way of joining the overlapped portions of a tensioned loop does not require an independent seal. Rather, with metal strap, notching jaw-type or punch-type mechanisms are forced against each side of the overlapped portions of the strap loop to cut a slit in, and to deform a portion of, the overlapped strap portions so that thereafter they may become nestably interlocked together. The third method is used to form the overlapping strap portions of a loop formed from plastic strap by positioning a heating assembly to cause interface melting between the portions so that both portions fuse together. A fourth method of joining overlapping strap portions of a loop can also be used on plastic strap or on plastic-coated metal strap and forms a friction-fused joint. The method for friction-fusing the overlapped portions of the strap loop requires that the overlapped portions of the loop be placed between an anvil and a movable weld head. The weld head is oscillated with a relatively small displacement at a relatively high frequency while it is in contact with the overlapping strap free end. This causes the strap free end to move back and forth against the overlapped portion of the strap and to generate heat by friction thereby effecting interface melting therebetween.
Present strapping machines which form any of these types of joints have a common drawback relating to the formation of the joint. In order to form these joints in the overlapped portions of the strap loop, an anvil or bearing member must be inserted between the package and the strap loop to provide a rigid bearing surface against which the overlapping strap portions are pressed. With machines that produce a slit-type joint or an independent crimped seal joint, such a bearing member may be an integral part of the notching or sealing jaw mechanism. Some types of machines also require that an anvil member be inserted between the package and the strap loop to effect proper tensioning prior to forming the joint. Regardless, any such anvil or bearing members prevent the strap loop from lying flat against the surface of the package at that point and therefore may introduce some slack into the loop when it is removed. However, owing to the flexibility of the strap, a tight loop can usually be obtained with large and/or slightly resilient packages. Unfortunately, though, with small packages and/or with rigid packages the amount of slack introduced into the strap loop by the inserted anvil or bearing member can be significant and can result in a loose strap loop when the anvil or bearing member is ultimately retracted from between the strap and the package. Accordingly, it would be desirable to provide a method and apparatus for tensioning and joining of the strap in a manner that does not require the insertion of an anvil member between the strap and the package.